FIG. 4 shows a conventional line driver device LT for transmitting line signals via a transmission line L. The line driver device LT has an output stage AS which provides a prescribable level for the line signals which are to be driven. The output stage AS is controlled by an operating-mode control unit BS, with the output stage AS being able to be switched to different modes by the operating mode control unit BS. The operating mode control unit BS receives, via a mode control bus BUS, control signals for controlling line driver device LT on the basis of external parameters. To protect the conventional line driver device LT, normally external protective elements SE are used which are connected between the transmission line L and the output stage AS.
Such protective elements SE ensure that, by way of example, overvoltages (which can be caused by a lightning strike and/or contact with the mains, for example) do not impair or even destroy the output stage AS or the entire line driver LT. Such external protective elements SE are prescribed in line with international standards such as K.20, K.45 (ITU_T) or GR.1089 (ANSI).
Conventionally, the protection provided by the protective elements SE is brought about essentially by transistors, diodes or similar components. Disadvantageously, such protective elements are expensive and such a protective device using the protective elements is highly complex. It is also a drawback that the conventional external protective elements are of complicated design.